1. Field of the Invention
The present invention relates to an inked ribbon advancement mechanism and more particularly to an advancement mechanism which incrementally advances an inked ribbon.
2. Description of the Prior Art
U.S. Pat. No. 3,986,594, entitled Serial Impact Calculator Printer, issued to Nicholas Kondur, Jr., on Oct. 19, 1976, teaches a wire matrix print head for a dot matrix printer that employs a plurality of electromagnetically actuated print wires converging forwardly through a wire guide member for termination at their leading ends in confronting and adjacent relation to the print medium. The trailing ends of the print wires are mounted within electromagnetic actuators for movement in a lengthwise direction to cause their leading ends to be driven into the print medium to form a series of even impressions or dots thereon.
A ribbon spool drive member coordinates the advancement of the inked ribbon with the print head travel and it is mounted on the print head to selectively engage ratchet teeth on a ratcheted ribbon spool at the end of each margin as the print head traverses the print medium. Each ribbon spool is constructed and arranged so that it exerts not only the proper tension on the inked ribbon on its passage between the print head and the print medium but also permits the selective advancement of the ribbon spool in response to its engagement by a ribbon spool drive member. The ribbon spool drive member is constructed and arranged so that it is selectively engageable with one ribbon spool at a time in order to advance the inked ribbon in one direction only until it is fully wound upon one of the ribbon spools and thereafter to be reversed to selectively engage the other ribbon spool in order to cause reverse travel of the inked ribbon.
The U.S. Pat. No. 3,825,103, entitled High-Speed Printer Having Improved Ribbon Driving, Reversing and Tensioning Mechanism, issued to Arthur F. Riley on July 23, 1974, teaches a high-speed impact printer which has an improved ribbon driving, reversing, and tensioning mechanism that is jam-proof, of compact-simplified construction, reliable and substantially maintenance free in operation. By being compact, the drive mechanism may be centrally located at the front of the print head so as to facilitate spool and/or inked ribbon replacement. The drive mechanism, through the use of two sets of pivotally mounted, two-stage biased planetary coupling gears, effects gradual rotational engagement and disengagement of the selectively coupled driving and driven gears of the drive mechanism. This results in minimal gear wear, and produces smooth, automated reversal of ribbon travel, while the latter is continuously maintained under uniform tension.
Because most dot matrix printers are of a high-speed nature, it precludes an incremental movement of the inked ribbon in view of the fact that normally there would be insufficient ink (or carbon) on the inked ribbon to allow repetitive impacting of the rods of a print head against common discrete areas during each index dwell period. Accordingly, the ribbon advancement in high-speed printers cannot be accomplished simply as a by-product of the type-bar mechanical motion of conventional typewriters. Rather, the ribbon spool drive mechanism must be of a type that slowly, but continuously, advances the inked ribbon along and between the aligned arrays of the rods of the print head, and under constant tension so as to maintain the inked ribbon in alignment therewith. Ribbon reversal, of course, must also be effected automatically in any high volume printing application. In order to efficiently utilize all of the ink (or carbon) on the inked ribbon, it is also very advantageous that the entire length of the inked ribbon be exposed to the rods of the print head impacting at some point in time during travel of the inked ribbon therepast in both directions.
U.S. Pat. No. 3,677,486, entitled Uniform Ribbon Feed Apparatus, issued to Campbell Findlay on July 18, 1972, teaches a ribbon feed apparatus in which a reciprocating pawl engages a ratcheted takeup ribbon spool. A supply ribbon spool feeds an inked ribbon to the ratcheted takeup ribbon spool incrementally with each driving stroke of the pawl. In order to achieve a nearly constant speed of ribbon feed, the apparatus combines a camming lug with the pawl which detects the changing diameter of the inked ribbon on the takeup spool and reduces the angular distance that the pawl is engaged with the ratcheted takeup ribbon spool as the ribbon diameter increases.
The inked ribbon is interposed between a print medium such as paper and a raised font on an impacting surface of one of the rods of a print head. The impact of the rods of the print head depletes the ink supply in the impacted area of the inked ribbon. Capillary action replenishes the impacted area from adjacent ribbon areas of the inked ribbon so that it can make another print of sufficient quality from the same area. To prevent reprinting on the impacted area of the inked ribbon used by a previous print, the inked ribbon is moved before the next print impact is undertaken. In determining the distance the inked ribbon should move between the impacts of the rods of the print head, one must consider the sufficiency of the reservoir of ink remaining to replenish the depleted area of the inked ribbon. Printing in close succession on the same area of the inked ribbon either delays the replenishment of the ink in the impacted portion of the inked ribbon or inhibits it altogether. However, moving the inked ribbon for relatively great distances at high speed between prints is wasteful and also presents timing and stress difficulties in a high print rate machine.